Manufacturers of electronic circuitry and components desire thermosettable compositions to prepare laminates, such as printed circuit boards, adhesives, such as die-attach adhesives, and encapsulants having high service temperatures. "High service temperature" polyepoxide thermosets as used herein refers to three dimensionally crosslinked materials having a glass transition temperatures (Tg) on the order of at least 130.degree. C., and preferably 180.degree. C. or higher.
Liquid thermosettable compositions which can be partially polymerized or cured to form an intermediate solid, semi-solid, or gel thermoplastic that is stable and storable at room temperature in the semi-solid form, and easily handled and capable of being formed into desired end product configurations prior to the formation of a final cured thermoset are desired. These intermediate semi-solid materials are referred to herein as "B stage" materials; the liquid compositions that can be formed into such storage stable, but flowable thermoplastic intermediates are referred to as "B stageable" and processes for forming the intermediate are referred to as "B staging".
"Polyepoxides" as used herein refer to a polymer or polymer mixture wherein at least one polymer contains at least two epoxide, ##STR1## functional groups.
Conventional polyepoxide thermosettable compositions containing curing agents, optionally with accelerators, for the curing or crosslinking reaction of the polyepoxide with the curing agent are known and are capable of forming high service temperature thermosets. When curing agents that are non-reactive with polyepoxides at room temperature are employed in combination with accelerators, that do not initiate or catalyze the curing reaction at room temperature, the compositions can be stored in the liquid state for some time prior to curing. However, because of the rapid exothermic reaction of conventional curing agents with polyepoxides upon heating to elevated temperatures, it is often extremely difficult or impossible upon the application of heat to prevent the composition from rapidly proceeding from the liquid form directly to the final three-dimensionally crosslinked thermoset without forming a desirable B stage thermoplastic. Further, since the final service temperature of the thermoset is a direct function of the temperature employed for the curing reaction, it is often difficult or impossible to obtain a very high service temperature thermoset (Tg greater than 150.degree. C.) because of the rapid onset of curing once the minimum elevated temperatures needed to initiate curing is reached. Complicating this situation is the fact that most liquid polyepoxide thermoset compositions are formulated in organic solvents. While the majority of the organic solvent used to formulate conventional polyepoxide thermosets volatilizes upon the application of elevated temperatures, some solvent can remain trapped in the thermoset resulting in a less than a 100% solid thermoset product having undesirable physical properties for certain applications. In addition, the presence of organic solvents in the liquid polyepoxide composition may present solvent removal, flammability and potential health problems. 2. Description of the Prior Art
Many acid anhydrides, having at least two acid groups per molecule, are known to be useful as curing agents for polyepoxides. Chapter 12 of The Handbook of Epoxy Resins, Lee and Neville, McGraw-Hill (1982) presents an excellent review of acid anhydride curing agents. The acid anhydrides which have been used as curing agents for polyepoxides include alicyclic anhydrides, aromatic anhydrides, chlorinated and brominated anhydrides, and some types of polymeric linear aliphatic anhydrides. Monomeric aliphatic anhydrides, however, have as a class been viewed as being unsuitable as polyepoxide curing agents since these anhydrides typically split to yield monofunctional carboxylic acid molecules. While each monocarboxylic acid molecules can react with a polyepoxide, monofunctional acids cannot react with polyepoxides to form the three dimensionally crosslinked network needed for a useful high service temperature thermoset. Certain linear polymeric anhydrides, such as those derived from the inter-molecular condensation of organic acids, can be used as polyepoxide curing agents since they are carboxyl-terminated polymers containing internal anhydrides, having the generalized structure HO[OC--(CH.sub.2 ).sub.x C00]H where n is at least 2, that provide the necessary difunctionality needed for the polyepoxide crosslinking reaction. Typical of these linear polymeric anhydrides useful with polyepoxides are polysebacic acid anhydride and polyazelaic acid anhydride. Linear polyanhydrides of the above type have also been employed as blends with certain monomeric anhydrides such as cycloaliphatic anhydrides, for example; hexahydrophthalic anhydride (HHPA), dodecylsuccinic anhydride (DDSA), an Nadic Methyl Anhydride.RTM. (NMA), a trademark of Allied Chemical Corporation, and aromatic anhydrides such as phthalic anhydride.
Linear monomeric anhydrides which are capable of polymerizing in situ in polyepoxide liquid thermosettable compositions, such as (meth)acrylic acid anhydride, have not, however, been reported as being useful by themselves as curing agents for polyepoxides.
U.S. Pat. No. 3,676,398 is directed to polymerizable diester-derivative monomers which can undergo subsequent vinyl polymerization with itself or another vinyl monomer or polymer to form crosslinked infusible materials. This patent discloses the formation of the polymerizable diester-derivative monomer by the reaction of a polyepoxide with an acryloyl carboxylic acid anhydride. The acryloyl carboxylic acid anhydride is illustrated as being the reaction product of two different carboxylic acids; one of which can be methacrylic acid. These anhydride reaction products, however, are not disclosed or suggested as including (meth)acrylic acid anhydride monomer or a material containing a (meth)acrylic acid anhydride monomer.
Itaconic and maleic anhydride are also well known curing agents for polyepoxides (See U.S. Pat. No. 4,503,200), however, since they exist as cyclic structures in monomeric form they are not disclosed or suggested in the art as being capable of forming useful B stage intermediates. Applicant shall demonstrate hereinafter by actual comparative examples that such cyclic anhydride monomers are not suitable in the practice of the present invention.
Numerous references exist disclosing the use of alicylic anhydride-containing polymers of styrene and maleic anhydride as curing agents for polyepoxides. Examples of such references include U.S. Pat. Nos. 2,781,333, 2,858,323 and 2,848,433, 3,732,332 and Japan Kokai No. 81-92911. These polymeric anhydrides are incapable, however, of forming a stable B stage intermediate because of their reactivity with polyepoxides.
Russian Patent No. 574,452 is related to the production of copolymers of (meth)acrylic esters and vinyl monomers. These copolymers are disclosed as being prepared by the free radical copolymerization of a vinyl monomer, specifically methyl methacrylate, with methacrylic acid anhydride and an epoxy compound, where the methacrylic acid anhydride and epoxy are present in an equimolar ratio. The patent teaches the formation of di/poly functional polymeric methacrylate. It is not, however, directed to a liquid B stageable thermoplastic or to a high service temperature polyepoxide thermoset. In addition, applicant's liquid polyepoxide composition does not require the need for methyl methacrylate or any other such monofunctional monomer in combination with the polyepoxide and (meth) acrylic anhydride curing agent to form the B stage or final thermoset.
It is an object of the invention to use a linear anhydride monomer which has the capability of homopolymerizing in situ in a composition, containing a polyepoxide without initially or significantly crosslinking with the polyepoxide, to form a B stage thermoplastic which is flexible, flowable, and stable upon storage, and which can be cured to form a high service temperature thermoset.
It is a further object of the invention to provide a solvent-free, B stageable thermosettable composition capable of yielding a high service temperature (100% solid) thermoset without the disadvantages of organic solvents.
It is an additional object of the present invention to provide a thermosettable B stageable composition which can be used in the fabrication of printed circuit boards, die-attach adhesives and encapsulants for electronic circuitry having a glass transition temperature higher than 150.degree. C., preferably higher than 180.degree. C. and most preferably higher than 225.degree. C. These objects of the invention and others will become apparent from the following detailed description and illustrative examples which follow.